1. Field of the Invention
In general, the present invention relates to wavelength division multiplexing using a plurality of signal lights having wavelengths different from each other. More particularly, the present invention relates to a light-transmitting apparatus applied to the wavelength division multiplexing and a wavelength-division-multiplexing (WDM) communication system.
2. Description of the Related Art
With extremely rapid popularization of the Internet all over the world in recent years, increasing the capacity of a communication system becomes important. In the initial WDM (wavelength division multiplexing), about 4 to 8 channels separated from each other by a wavelength interval corresponding to a frequency interval of about 200 GHz are multiplexed. In recent years, however, multiplexing of about 16 to 40 channels separated from each other by a wavelength interval corresponding to a frequency interval of about 100 GHz is normal multiplexing. Furthermore, multiplexing of up to 100 channels separated from each other by a wavelength interval corresponding to a frequency interval of about 50 GHz is taken into consideration. In a first line line terminal equipment in a wavelength-division-multiplexing communication system, a receiving unit (TRIB (tributary)) of each channel receives a signal from a transmission line and converts the signal into an optical signal with a proper wavelength. Then, the optical signal is subjected to WDM in a multiplexer (MUX) prior to transmission through an optical transmission line. The WDM signal light passes through the optical transmission line and relay apparatuses and is received by a second line terminal equipment serving as a partner equipment.
A demultiplexer DMUX employed in the second line terminal equipment demultiplexes the WDM signal light into components having wavelengths different from each other, and transmits each of the wavelength components from a transmitting unit (TRIB) of a channel for transmission of the wavelength component being transmitted to a proper transmission line. Each channel of the first line terminal equipment is associated with the second line terminal equipment's channel for receiving data transmitted by the channel of the first line terminal equipment on a 1-with-1 basis. In addition, each TRIB of the transmitting channel in the first line terminal equipment is also associated with the second line terminal equipment's TRIB for a receiving channel provided for transmission of an optical signal having the transmitting channel on a 1-with-1 basis too. Thus, in order to avoid incorrect connection to a TRIB cable for the channel of the second line terminal equipment, typically, an ID for the channel is inserted into the header of an STM-64 frame of each wavelength component and transmitted along with the WDM signal. The TRIB employed in the second line terminal equipment examines the ID of a received frame to form a judgment as to whether or not the ID is correct and outputs an ID-error signal in case the ID is found incorrect. With the conventional wavelength interval, the demultiplexer employed in the second line terminal equipment is capable of demultiplexing the WDM signal light completely so that, in case the optical signal of a wavelength component of the first line terminal equipment is missed, the second line terminal equipment will not incorrectly detect another optical signal as the missed optical signal. In addition, since the number of wavelengths or the number of channels is small, it is easy to verify whether or not a normal signal has been exchanged by monitoring signals for all the wavelengths. Furthermore, there is also no problem raised in generation of an alarm based on only information on the power of a light.
Nevertheless, the conventional light-transmitting apparatus has the following problems.    1: FIGS. 23A to 23C are each a diagram showing normal reception. On the other hand, FIGS. 24A to 24C are each a diagram showing reception with the signal of channel 2 missed. FIG. 23A is a diagram showing a spectrum of a received signal. FIG. 23B is a diagram showing a spectrum of a received signal of channel 2. FIG. 23C is a diagram showing a post-amplification spectrum of a received signal of channel 2. FIG. 24A is a diagram showing a spectrum of a received signal with the signal of channel 2 missed. FIG. 24B is a diagram showing a spectrum of a received signal of channel 2. FIG. 24C is a diagram showing a post-amplification spectrum of a received signal of channel 2. The spectrums shown in FIGS. 23A and 24A each exhibit differences in level among channels. These differences are caused by differences in attenuation among channel signals provided that a relay apparatus 28 or the like transmits optical signals of all channels at a uniform level. In turn, the differences in attenuation among channel signals are caused by dependence on channel wavelengths which are different from each other.
As the number of different wavelengths is increased and, hence, the wavelength interval decreases as shown in FIGS. 23A and 24A, the demultiplexer of the second line terminal equipment is conceivably no longer capable of effectively demultiplexing a WDM signal light due to limitation on a characteristic of the demultiplexer. In such a condition on the receiving equipment, a signal of a particular channel is not completely filtered out from a signal of a channel adjacent to the particular channel, remaining as a residual signal in the adjacent channel. In the case of the spectrum shown in FIG. 23B, for example, signals of channels 1 and 3 remains as residual signals in channel 2. In the case of the spectrum shown in FIG. 24B, on the other hand, a residual signal remains in channel 3. Thus, in the case of normal reception like the one shown in FIGS. 23A to 23C, no problem is raised since main signals of all channels are amplified as shown in FIG. 23C. In the case of the abnormal reception wherein the main signal of channel 2 is missed as shown in FIGS. 24A to 24C, on the other hand, it is quite within the bounds of possibility that the main signal of channel 2 remains as a residual signal in channel 3. The residual signal is incorrectly detected as a main signal of channel 3 and inevitably amplified to a certain level on the receiving equipment as shown in FIG. 24C. In a transmitting line terminal equipment, however, an ID number is assigned to each channel and stored in the header of a frame of an optical signal transmitted through the channel as described above so that the TRIB on the receiving line terminal equipment is capable of comparing the ID stored in the header with an ID assigned to the channel in order to detect an ID error, that is, an ID mismatch between the optical signal and the channel for transmission of the signal, and, hence, to avoid a main signal from being incorrectly recognized. However, an ID error is detected only to point out incorrect connection of a cable. Even in the case of a missed main signal, however, an ID error may also be detected due to the missed signal remaining as a residual signal in an adjacent channel as described above. For example, the missed signal of channel 2 remains as a residual signal in channel 3 and is amplified in the TRIB as a main signal as shown in FIG. 24C. Thus, an ID error may indicate incorrect connection of a cable or a missed optical signal. As a result, an ID error indicating incorrect connection of a cable cannot be distinguished from an ID error indicating a missed optical signal, causing some fear of confusion in maintenance work.    2: As the number of different wavelengths and, hence, the number of channels assigned to the different wavelengths increases, it is also becoming no longer easy to individually monitor signals having the wavelengths different from each other in order to confirm normal reception of the signals. That is to say, there is raised a problem of maintainability. Traditionally, only the power of each transmitted signal light is detected and an alarm is issued to indicate a problem in the detected power.
FIG. 25A is a diagram showing a spectrum of a normal-power and normal-OSNR (Optical Signal Noise Ratio) optical signal transmitted through transmission channel 1 of the TRIB of the first line terminal equipment. FIG. 25B is a diagram showing a power spectrum of a normal-power and normal-OSNR optical signal transmitted through transmission channel 2 of the TRIB of the first line terminal equipment. FIG. 25C is a diagram showing a power spectrum of a transmitted optical signal obtained as a result of a synthesis of the signals transmitted through transmission channels 1 and 2. FIG. 26A is a diagram showing a power spectrum of a normal-power and normal-OSNR (Optical Signal Noise Ratio) optical signal transmitted through transmission channel 1 of the TRIB of the first line terminal equipment. FIG. 26B is a diagram showing a power spectrum of a normal-power and abnormal-OSNR optical signal transmitted through transmission channel 2 of the TRIB of the first line terminal equipment. FIG. 26C is a diagram showing a power spectrum of a transmitted optical signal obtained as a result of a synthesis of the signals transmitted through transmission channels 1 and 2. An OSNR is a ratio of the level of a main signal transmitted through a channel to the level of a noise introduced to the main signal during the transmission through the channel. A normal OSNR is an OSNR having a value at least equal to a predetermined one. On the other hand, an abnormal OSNR is an OSNR having a value smaller than the predetermined one.
Now, consider a condition represented by the power spectrum shown in FIG. 26B in which the OSNR for transmission channel 2 is abnormal due to a high level of a noise such as ASE (Amplified Spontaneous Emission). The high level of the noise can also be caused by typically degrade of a cable connected to the TRIB. By merely taking the power of the light into consideration, a total power of the main signal transmitted through the transmission channel and the noise introduced during the transmission will be detected so that the signal will be determined to be normal even in such a condition. This is because a total level representing the sum of the levels of the noise and the main signal indicates a power at least greater than a predetermined value. Thus, the multiplexer MUX will synthesize and transmit the main signal. As a result, the spectrum after the synthesis exhibits actually a small power for the main signal of transmission channel 2 as shown in FIG. 26C even if the total level has been determined to be normal.
Assume that the second line terminal equipment is capable of correctly accepting the main component of an optical signal with an abnormal OSNR. Even in this case, since the wavelength interval between noises also becomes shorter, a noise mixed with a main signal of a particular transmission channel is added to a noise mixed with a main signal of a transmission channel adjacent to the particular transmission channel, resulting a big resultant noise in the adjacent transmission channel. Thus, the difference between the resultant noise and the main signal becomes very marginal, causing some fear of incorrect detection of the main signal. Such incorrect detection of the main signal will generate an ID error. When an alarm caused by an ID error is output in the second line terminal equipment in such a case, however, it will be difficult to determine the cause of the ID error. As a result, there is raised a maintenance problem.